The present invention relates to an optical information recording medium for recording and reproducing information, and more particularly, to an optical information recording medium which can suppress warpage caused by a change in environments or variance with time.
As an example optical information recording medium for recording and reproducing information, a thin disk type has been known. FIG. 6(a) is a plan view of such an optical information recording medium and FIG. 6(b) is a side elevation thereof.
FIG. 7 is a schematic cross section showing an arrangement of a conventional optical information recording medium 110.
As shown in FIG. 7, the conventional optical information recording medium 110 includes a disk (see FIG. 6(a)) substrate 120 made of polycarbonate or the like, on which a single- or multi-layer thin film layer 140 comprising thin films, such as dielectric films 141 and 143 (silicon nitride, etc.), a recording film 142 (TbFeCo, etc.), and a reflecting film 144 (Al, etc.), is formed by means of sputtering or the like.
Also, a thin film protecting film 150 made of a resin film or the like is formed on the thin film layer 140, and a substrate protecting film 130 made of resin or the like is formed on the light incident surface of the substrate 120.
The thicknesses of these films and layers are as follows: the thickness of the substrate 120 is approximately 1.2 mm; the thickness of the single- or multi-layer thin film layer 140 formed by means of sputtering or the like is 10-30 nm; the thickness of the thin film protecting film 150 is 1-30 xcexcm; and the thickness of the substrate protecting film 130 is 1-30 xcexcm. Thus, the substrate 120 made of polycarbonate occupies most of the optical information recording medium 110 in thickness.
The rigidity of the optical information recording medium 110 depends almost entirely on the substrate 120, and because the substrate 120 is sufficiently thick, deformation caused by a change in environments (temperature and humidity changes) is quite small. For this reason, a balance of stress and a bending moment of each layer has not been considered generally in most of the cases.
However, there has been a demand to further increase recording and reproducing density of the optical information recording medium, and the substrate has been made thinner (for example, the thickness is now reduced to 0.6 mm from 1.2 mm) to suppress the occurrence of aberration. As a result, the rigidity of the optical information recording medium is reduced, and larger deformation occurs due to stress produced in each layer forming the optical information recording medium with a change in environments (temperature and humidity changes), thereby posing a problem that information can not be readily recorded and reproduced. Thus, there has been an increasing need for an optical information recording medium which can maintain good performance in response to a change in environments even if its rigidity is reduced by employing a thinner substrate.
Japanese Laid-open Patent Application No. 195745/1992 (Japanese Official Gazette, Tokukaihei No. 195745, published on Jul. 15, 1992) discloses a technique of suppressing deformation of the optical information recording medium, by which a warpage preventing dielectric film is provided on the back surface (the surface on which the thin film layer is not formed) of the substrate (prior art {circle around (1)}).
FIG. 8 is a cross section showing an arrangement of the above optical information recording medium (prior art {circle around (1)}). In FIG. 8, like components are labeled with like reference numerals with respect to FIG. 7 for ease of explanation. As shown in FIG. 8, a dielectric layer 160 is provided on the light incident side of the substrate 120 made of polycarbonate, so that the same expansion coefficient is given to the recording film 142 and dielectric layer 160 which are provided respectively at the both sides of the transparent substrate 120. Consequently, because the optical information recording medium has a symmetrical structure with respect to the substrate 120, warpage of the optical information recording medium can be prevented.
Also, Japanese Laid-open Patent Application No. 64119/1998 (Japanese Official Gazette, Tokukaihei No. 10-64119, published on Mar. 6, 1998) discloses that, by making a thin film protecting film thicker (30-50 xcexcm), warpage occurring with increasing temperatures can be prevented (prior art {circle around (2)}).
Further, Japanese Laid-open Patent Application No. 364248/1992 (Japanese Official Gazette, Tokukaihei No. 4-364248, published on Dec. 16, 1992) proposes an optical information recording medium which can solve problematic warpage caused by a humidity change. This optical information recording medium includes, as shown in FIG. 9, a thin film protecting film 150, a thin film layer 140, a substrate 120, a substrate protecting film 130, and in order to solve the problem, it additionally includes a moisture permeation preventing film 170 made of SiO2, AlN, etc. between the substrate 120 and substrate protecting film 130 (prior art {circle around (3)}). In FIG. 9, like components are labeled with like reference numerals with respect to FIGS. 7 and 8 for ease of explanation.
However, according to the technique disclosed in Japanese Laid-open Patent Application No. 195745/1992 supra (see FIG. 8, prior art {circle around (1)}), the dielectric layer 160 has to be provided on the light incident side of the substrate 120 by means of sputtering or the like. In this case, the manufacturing procedure includes forming the thin film layer 140 on one surface of the substrate 120, turning over the substrate 120, and forming the dielectric layer 160 on the opposite surface. Thus, not only the manufacturing procedure becomes complex, but also expensive manufacturing facility is required, thereby posing a problem that the manufacturing costs are undesirably increased.
Also, the technique (prior art {circle around (2)}) of Japanese Laid-open Patent Application No. 64119/1998 supra poses a problem that the thin film protecting film is so thick that it can not be readily formed. In addition, in case that the optical information recording medium is a magneto-optical recording medium, for example, in order to turn an applied magnetic field inversely at a high speed while information is being recorded, it is preferable to approximate the thin film layer to magnetic field generating means. However, a too thick thin film protecting film can cause problematic deterioration of magnetic characteristics.
Further, the technique of Japanese Laid-open Patent Application No. 364248/1992 supra (see FIG. 9, prior art {circle around (3)}) demands the moisture permeation preventing film 170 made of SiO2, AlN, etc. to be provided on the light incident side of the substrate 120 by means of sputtering or the like. In this case, the manufacturing procedure includes forming the thin film layer 140 on one surface of the substrate 120, turning over the substrate 120, and forming the moisture permeation preventing film 170 on the opposite surface. Thus, not only the manufacturing procedure becomes complex, but also expensive manufacturing facility is required, thereby posing a problem that the manufacturing costs are undesirably increased.
It is therefore an object of the present invention to provide an optical information recording medium which can prevent deformation (warpage) caused by temperature and humidity changes and be readily manufactured.
In order to fulfill the above and other objects, an optical information recording medium of the present invention is characterized by being furnished with:
a thin film layer, formed on a substrate, for recording and reproducing information; and
a thin film protecting film, formed on the thin film layer, for protecting the thin film layer,
a neutral plane of deformation in a thickness direction caused by a temperature change being present in a vicinity of the thin film layer.
According to the above arrangement, the optical information recording medium has a multi-layer structure in which the thin film layer and thin film protecting film are formed on the substrate.
If the substrate is made thinner in the optical information recording medium having such a multi-layer structure, the rigidity is reduced, thereby posing a problem that warpage occurs in the thickness direction toward the thin film protecting film in response to a temperature change.
To solve the above problem, a conventional optical information recording medium employing a thin substrate is additionally provided with a warpage preventing dielectric layer. This solution, however, raises another problem that, by providing the additional layer, the number of manufacturing steps and manufacturing costs are undesirably increased. In particular, because the dielectric layer is formed on the opposite side (the side opposite to the side where the thin film layer is formed) of the substrate, the substrate has to be turned over after the thin film layer is formed. Accordingly, not only the manufacturing procedure becomes complex, but also expensive manufacturing facility is required, thereby increasing the manufacturing costs.
In contrast, according to the arrangement of the present invention, the neutral plane of deformation in the thickness direction caused by a temperature change is present in the vicinity of the thin film layer. In other words, bending moments applied on the thin film layer from the substrate side and thin film protecting film side are substantially cancelled out with each other.
More specifically, warpage of the optical information recording medium in the direction toward the thin film protecting film is caused by a bending moment applied on the thin film layer from the substrate side. Thus, according to the arrangement of the present invention, the bending moment applied on the thin film layer from the substrate side is cancelled out with a bending moment applied thereon from the thin film protecting film side, and the vicinity of the thin film layer serves as the neutral plane of deformation in the thickness direction. Hence, the optical information recording medium of the present invention causes warpage neither in the thickness direction nor in the opposite direction.
Consequently, different from the conventional arrangement, the additional warpage preventing dielectric layer can be omitted, thereby eliminating the problem that the manufacturing procedure becomes complex and the manufacturing costs are increased.
In order to fulfill the above and other objects, another optical information recording medium of the present invention is characterized by being furnished with:
a thin film layer, formed on a substrate, for recording and reproducing information;
a thin film protecting film, formed on the thin film layer, for protecting the thin film layer; and
a substrate protecting film, formed on the substrate on a surface opposite to a surface where the thin film layer is formed, for protecting the substrate,
a moisture permeation degree of the substrate protecting film being smaller than a moisture permeation degree of the thin film protecting film.
In case of the optical information recording medium in which the thin film layer and thin film layer protecting film are formed on one side of the substrate and the substrate protecting film on the other side, only a slight quantity of water is absorbed from the external and reaches the substrate in the thin film protecting film side because of the thin film layer interposed therebetween, whereas water readily reaches the substrate in the substrate protecting film side. Thus, there is a problem that a volume change occurs locally on the substrate in response to a humidity change, thereby causing warpage of the optical information recording medium.
In order to solve such a problem, a conventional optical information recording medium is additionally provided with a moisture permeation preventing film which can prevent warpage caused by a humidity change. This solution, however, raises another problem that, by providing the additional layer, the number of manufacturing steps and manufacturing costs are undesirably increased. In particular, because between the substrate and substrate protecting film, the substrate has to be turned over after the thin film layer is formed. Accordingly, not only the manufacturing procedure becomes complex, but also expensive manufacturing facility is required, thereby increasing the manufacturing costs.
In contrast, according to the arrangement of the present invention, a moisture permeation degree of the substrate protecting film is smaller than that of the thin film protecting film. Hence, because absorption of water from the substrate protecting film side can be reduced, warpage of the optical information recording medium caused by a humidity change can be suppressed.
Consequently, different from the conventional arrangement, the additional moisture permeation preventing film can be omitted, thereby eliminating the problem that the manufacturing procedure becomes complex and the manufacturing costs are increased.